Cook Island Aquatic Reserve, gazetted in 1998, is a compact volcanic outcrop about 600 m off Fingal Head that punches well above its size as a dive site. It is the closest offshore reef to the Tweed/Coolangatta coast and, because it sits inshore in the lee of Fingal Head, it can usually be dived when the exposed reefs further out are blown out by swell, which makes it the workhorse site for every operator on this stretch of coast. The island is ringed by roughly a dozen named moorings and the Northern Ledge is the pick of them: a boulder-strewn wall that starts around 6 m on top and steps down through a series of ledges, swim-throughs and sand-bottomed gutters to about 22 m on its outer face.

The rock is thickly encrusted with sponges, gorgonian fans and hard and soft corals, a legacy of the site's position where the warm, southward-flowing East Australian Current meets cooler temperate water, and the marine life reflects that mix. Green and loggerhead turtles are almost guaranteed, resting under ledges or grazing on algae on the boulders, and it is common to see three or four on a single dive. Wobbegong sharks lie tucked into overhangs, moray eels work the cracks at night, and leopard (zebra) sharks cruise the sand in the warmer months alongside eagle rays and the occasional manta.

In winter, grey nurse sharks move onto the deeper ledges to join the residents, and lionfish, nudibranchs and cuttlefish round out an unusually busy critter list for a site this shallow and easy. Because depths rarely exceed 20 m and the topography is gentle, Cook Island is comfortably within Open Water limits and is the default first ocean dive, first shark encounter and first turtle dive for hundreds of divers a year passing through Coolangatta and Tweed Heads. It also snorkels well on calm days.

The only real downside is popularity — on a fine weekend several boats can be on the moorings at once — and a drop in visibility for a few days after heavy rain pushes silt out of the Tweed River. For anyone starting out on the northern NSW/southern Gold Coast coast, this is the site to learn on.

Boat access only, from Kirra or Currumbin on the Gold Coast side (10–20 minutes) or by private boat launched at Fingal Head or Jack Evans Boat Harbour on the Tweed River, then a short run south past Fingal Head. Cook Island sits close inshore in relatively protected water, so it is the fallback site when the open reefs (Nine Mile) are blown out.

The Tweed/Coolangatta coast is exposed to south-easterly and easterly swell year-round; charters generally want under 1.5 m swell and light winds. A strong East Australian Current push (common November–May) can add 1–2 knots of southerly drift on the outer reefs, turning some sites into serious drift dives. The Tweed River bar and river mouth are tide- and swell-dependent and can close to small boats in a big easterly swell.

Visibility on the inshore reefs drops for days after heavy rain flushes the Tweed River; the offshore reef and shark sites are less affected. Winter (June–September) gives the clearest, calmest water and the grey nurse shark season; summer is warmer with more current and better odds of leopard sharks and mantas.

Descend the mooring line onto the northern boulder wall around 10–12 m, work north-east along the ledge system to the deeper outcrops at 18–22 m looking for turtles and wobbegongs in the crevices, then shallow up over the boulders back to the mooring for the safety stop at 5 m.

Gold Coast Dive Adventures (Currumbin) and Kirra Dive (Coolangatta) run Cook Island most days as the easy option; Sundive and Byron-based charters occasionally include it on northern trips.